The following were taken from some of Cordain's writings.

                                         Excessive intake of cereals
(grains). The effects of cereal phytate in limiting mineral absorption are well-known
and thereby unfavorably impact calcium uptake, though this is often not noted
when considering the overall picture of calcium balance in relation to the
potential for osteoporosis. In addition, the acidifying effects of cereals upon
the urine [Barzel and Massey 1998] (via the kidney), cause calcium carbonates
from bone mineral reserves to be used to buffer the slight metabolic acidosis
caused by cereals.
Sodium chloride (salt) consumption. One is that Stone-Age men and women did
not consume supplemental dietary sodium chloride (salt), which like protein can
also cause increased calciuresis (calcium excretion) [Nordin et al. 1993] and
loss of bone mass [Devine et al. 1995]. Because the kidney must obligatorily
excrete calcium with sodium [Nordin et al. 1993], high levels of dietary
sodium are now generally recognized to be the single greatest dietary risk factor
for osteoporosis [Matkovic et al. 1995; Devine et al. 1995; Cappuccio 1996]. It
should go without saying that in this context, "high" levels of dietary
sodium are simply normal levels in Western societies.
For further info go to the following url's
http://www.beyondveg.com/cordain-l/prot-calc/prot-calcium-loss-1a.shtml
http://www.beyondveg.com/nicholson-w/hb/hb-interview1a.shtml